<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div {
            width: 20 0px;
            height: 200px;
            background-color: cadetblue;
            margin-bottom: 10px;
        }
        button {
            margin-left: 10px;
        }
        .change-border {
            border-radius: 50%;
        }
    </style>
</head>
<body>
    <div class="change"></div>
    <button class="btn">变成圆形</button>
    <button class="btn">变回方形</button>
    <script>
        var box = document.getElementsByClassName('change')
        var btns = document.getElementsByClassName('btn')
        btns[0].onclick = function () {
            box[0].classList.add('change-border')
        };
        btns[1].onclick = function () {
            box[0].classList.remove('change-border')
        };
    </script>
</body>
</html>